"Our position against the use of human embryos is rooted as much in reason as it is in faith," according to the Rev. Frank Pavone, who heads the Catholic pro-life group Priests for Life. "What the pope is saying is that reason itself tells us that, just as you can't use a human person by enslaving him or her, neither can you use a much smaller, younger human person for the purposes of research." (link
This appeal to the Kantian 'mere means' principle is one common in the anti-cloning of embryos for medical research camp. I think Frances Kamm has a nice article in the Boston Review that deals with this. Nothing groundbreaking in her article I suppose. She appeals to the distinction between the ways in which principles cover potential humanity and those with humanity in the moral sense and offers a nice example.
Suppose that medical technology enabled us to harvest frozen embryos for parts that we could use for repairing damaged embryos that could then be used for reproduction which, in the course of things, would effectively destroy the first embryo. Should wannabe parents be allowed to do this? (Unscientific anonymous polling of students suggest that over 90% think 'Yes'). She then runs the same example but using pairs of infants and we get 100% response that this is impermissible. Anyway, I regard these results as interesting since the majority of my students regard themselves as pro-life (maybe 60% or more) and a significant portion of them (maybe 40% or so) will say that they think that embryos and fetuses are the moral equivalents of persons (I've said this before in response to one of Jeremy's remarks, but I think this shows that these people don't really believe what they think they do about the moral status of embryos, but that's just me).